Physiologically, human blood sugar levels are maintained at a constant level within certain boundaries (60 . . . 120 mg/dl fasting at rest). Blood sugar levels are influenced by the following four hormones via the hormonal feedback control system:                Insulin (formed in the beta cells of the pancreas)        Glucagon (formed in the alpha cells of the pancreas)        Adrenaline (formed in the adrenal medulla)        Glucocorticoids (e.g. cortisol, formed in the adrenal cortex).        
While glucagon increases blood sugar levels by reducing glycogen reserves in the liver, insulin lowers blood sugar levels by transferring the sugar from the blood into the various body cells. Adrenaline (during sport) and cortisol (during stress) have a slower effect and also increase blood sugar levels. Consequently, insulin is the only hormone which reduces blood sugar levels. FIG. 1 shows schematically how insulin and glucagon affect blood sugar levels.
Diabetes mellitus is characterised by a relative or absolute lack of insulin, e.g. due to the destruction of beta cells as a result of an auto-immune disease (Type I) or due to the loss of insulin efficacy (insulin resistance, Type II). Consequently, the control of blood sugar levels is hampered, resulting in an increase in the mean blood sugar level. External glucose input (e.g. intake of food) or the internal supply of glucose (adrenaline, cortisol) cannot be adequately corrected for diabetics, allowing this to be used in the diagnosis of diabetes mellitus.
As part of modern standard treatment methods, blood sugar levels are determined with the aid of blood from a finger taken one or more times a day employing enzymatic chemical methods using a test strip. FIG. 2 shows a schematic representation of such a testing device.
In addition to dietary measures (compensatory reduction of external sugar intake), attempts can be made to regulate blood sugar levels using medication (e.g. using sulfonylurea to stimulate increased insulin production) or by means of subcutaneous injection of insulin into physiological tracts. Injection aides or mechanical pumps exist for continuously injecting insulin.
The blood sugar levels of most diabetics is poorly regulated as a result of the often insufficient frequency of measurement and the lack of an opportunity to respond adequately to disruptions. In the case of chronic to high blood sugar levels, subsequent damage occurs, particularly to nerves and vessels, which can lead to drastic consequences (arteriosclerosis, heart attack, amputations, blindness, obligatory dialysis due to renal failure etc.).
Various treatment approaches are currently at an experimental stage, aiming to close the blood glucose control loop adequately with acceptable control quality. One differentiates between “biological” and “technical” approaches.
With the biological approach an attempt is made to utilise the natural functions of donated islet cells as sensors and actuators. Entire pancreas transplants (approx. 100 p.a. in Germany) and the transplantation of isolated donor islet cells (approx. 20 p.a. in Germany) are used clinically.
Both these procedures have a limited prospect of success due to the approximately constant number of donors which is to low, the necessity of immune suppression with its risk of infection, the risk of the genesis of malignant tumours and high medication costs.
The use of the unlimited supply of animal islet cells is restricted by the even stronger immune reaction.
The technical approaches attempt to close the defective control loop by employing technical means.
It is known that insulin pumps (e.g. roller or piston pumps) can be used as actuators which can be operated externally as well as in the form of implants. FIG. 3 shows an insulin pump attached to a patient.
The core problem for all technical treatment approaches is the reliable, comfortable and painless determination of blood sugar levels as continuously as possible. It is known that chemical procedures can be used as blood sugar sensors, e.g. enzymatic procedures such as glucose oxidase (GOD) reaction, in some cases in combination with ultrafiltration/microdialysis, or electrocatalytic procedures (direct oxidation of glucose on platinum electrodes). It is also known that these methods can be miniaturised and sometimes implanted. For instance, implantable prototypes for microdialysis sensors with GOD reaction exist (“Ulm “Sugar Watch System””). The main difficulty for all chemical sensor approaches is their short service life (<3 weeks).
A device of this type known from U.S. Pat. No. 4,975,581. In order to measure the blood sugar level, the catheter is externally inserted into an arm vein, whereby provision is made for a connection to an insulin pump which introduces into the body the amount of insulin calculated on the basis of an evaluation of the light dispersed by the blood.
Following this procedure, the catheter must be removed from the arm vein. This device has the disadvantage that the injection area can become inflamed from the catheter having to be inserted frequently. Moreover, the mobility of the patient is still severely restricted with this method. A corresponding procedure involving the blood sugar concentration being determined by evaluating the polarisation of light is known from DE 195 40 456 C2.
U.S. Pat. No. 4,704,029 makes known a combinatory approach for measuring blood sugar content by evaluating the absorption, reflection and polarisation of light by the blood. In principle, this device is implantable. However, a major problem is the fact that the measurement interface becomes so contaminated from deposition that the measured values cannot be used.
DE 37 36 092 A1 makes known a measuring device for continually determining the concentration of the blood sugar content in a measurement cuvette, whereby the blood sugar concentration is determined by determining the optical rotation by polarizers and analysers. A corresponding device using the analysis of the reflection of the light by the blood is known from WO 97/28437.
WO 91/18548 makes known a device whereby the blood sugar level is determined externally through the skin and which involves two infrared wavelengths being transmitted and received, with the absorption of the wavelengths being analysed. However, the accuracy of these non-invasive “in vivo” measurements is not sufficient for clinical usage owing to the absorption by the skin and mucous membrane cells. A procedure of this nature for evaluating a test beam and a reference beam is known from U.S. Pat. No. 5,146,091.